In general, a natural knee is formed by the two condyles at the bottom part of the femur, the lower surfaces of which bear upon the correspondingly shaped upper surface plateaus of the tibia. Connections between the portions of the knee are provided by means of ligaments which also provide joint stability and help to absorb stresses applied to the knee. These knee members are normally subjected to relatively large forces in the course of supporting a major portion of the body weight of a person. Movement of a normal knee is very complex and is not simply a pivotal or rotational movement.
There are two main types of prosthetic knees. The first type is an articulated device in which the femoral and tibial prosthetic components are mechanically linked or constrained to each other by means of a hinge therebetween. The second type is an unlinked device which provides articulation between the femoral implant surfaces and the tibial implant surfaces. Some tibial components are made in a single one-piece configuration, while others are made of a separate tibial bearing which mates with a tibial support member or reinforcing tray. With some of these multi-piece tibial components, the tibial bearing is fixedly secured to the tray portion and thus does not move with respect thereto. U.S. Pat. Nos. 5,047,057 and 4,808,185 show such fixedly secured multi-piece tibial components. Other multi-piece tibial components are designed to allow movement between the tibial bearing and the tibial tray and will be referred to as mobile bearing knees. Thus, such mobile bearing knees allow both movement between the bearing and tray, and movement (articulation) between the femoral component and tibial bearing.
An example of a prior art mobile bearing knee is disclosed in European patent application to Walker, Publication No. 498 586 A1. The tibial component of Walker includes a plastic bearing component supported on a metal platform for sliding movement in the anterior-posterior direction. The mating surface between the bearing and the platform has a sagittal curve to it. The curve on this mating surface is in the anterior-posterior direction, while this mating surface is flat in the medial-lateral direction. This structure disclosed in Walker does not allow for gradual stiffening against rotation at the bearing/platform interface when the femoral and tibial components are rotated relative to one another.
Another prior art mobile bearing prosthetic knee is disclosed in U.S. Pat. No. 4,224,696 to Murray et al. The prosthetic knee of Murray includes a tibial implant having a continuous concave spherical surface engaging a meniscal plate. The Murray et al. knee is free to rotate without resistance.
Other examples of mobile bearing knees are disclosed in the following U.S. Pat. Nos.: 5,171,283; 5,080,675; 4,883,488; 4,728,332; 4,586,933; 4,568,348; 4,353,136.
The present invention is directed to a mobile bearing prosthetic knee, wherein it is desired to create a tibial implant with two specially shaped condyle depressions whereby movement and control of the knee is enhanced.